For sauce:Heat oil in large saucepan over medium heat.Add onion and garlic and cook until soft, stirring often, about 5 minutes.Add wine and grapes.Simmer over medium heat until almost all liquid is evaporated, stirring occasionally, about 20 minutes.Add broths.Boil over medium-high heat until reduced to 2 scant cups, about 40 minutes.Strain into measuring cup, pressing on solids to release liquid; discard solids in strainer. Season with salt and pepper.DO AHEAD:Can be made 3 days ahead. Cover; chill.

Preheat oven to 350°F. Place potatoes on small baking sheet.Drizzle with 3 teaspoons oil, sprinkle with salt and pepper, and toss to coat.Roast until tender, about 25 minutes.Cool slightly.Cut potatoes crosswise into 3/4inch pieces; transfer to medium bowl.

Heat 2 teaspoons oil in large skillet over medium-high heat.Add cauliflower and sauté until brown in spots, about 3 minutes.Add 1/4 cup water, cover, and cook until tender, adding more water by tablespoonfuls as needed, about 3 minutes longer.Transfer cauliflower to bowl with potatoes.Wipe skillet clean.Add remaining 2 teaspoons oil to same skillet.Add brussels sprouts and sauté until brown in spots, about 3 minutes.Add 1/4 cup water, cover, and simmer until tender, adding more water by tablespoonfuls as needed, about 3 minutes longer.Transfer to bowl with cauliflower and potatoes.DO AHEAD:Curry butter and vegetables can be made 2 hours ahead.Let stand separately at room temperature.

For gremolata and lamb:Bring verjus and sugar to boil in small saucepan, stirring until sugar dissolves.Reduce heat to medium-high and boil until reduced to 1/4 cup, about 5 minutes. Cool.

Preheat oven to 250°F.Place grapes on rimmed baking sheet lined with parchment or foil.Roast until reduced in size by half, about 1 1/2 hours. Cool.